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const perf =
  typeof performance === 'object' &&
  performance &&
  typeof performance.now === 'function'
    ? performance
    : Date

const hasAbortController = typeof AbortController === 'function'

// minimal backwards-compatibility polyfill
// this doesn't have nearly all the checks and whatnot that
// actual AbortController/Signal has, but it's enough for
// our purposes, and if used properly, behaves the same.
const AC = hasAbortController
  ? AbortController
  : class AbortController {
      constructor() {
        this.signal = new AS()
      }
      abort(reason = new Error('This operation was aborted')) {
        this.signal.reason = this.signal.reason || reason
        this.signal.aborted = true
        this.signal.dispatchEvent({
          type: 'abort',
          target: this.signal,
        })
      }
    }

const hasAbortSignal = typeof AbortSignal === 'function'
// Some polyfills put this on the AC class, not global
const hasACAbortSignal = typeof AC.AbortSignal === 'function'
const AS = hasAbortSignal
  ? AbortSignal
  : hasACAbortSignal
  ? AC.AbortController
  : class AbortSignal {
      constructor() {
        this.reason = undefined
        this.aborted = false
        this._listeners = []
      }
      dispatchEvent(e) {
        if (e.type === 'abort') {
          this.aborted = true
          this.onabort(e)
          this._listeners.forEach(f => f(e), this)
        }
      }
      onabort() {}
      addEventListener(ev, fn) {
        if (ev === 'abort') {
          this._listeners.push(fn)
        }
      }
      removeEventListener(ev, fn) {
        if (ev === 'abort') {
          this._listeners = this._listeners.filter(f => f !== fn)
        }
      }
    }

const warned = new Set()
const deprecatedOption = (opt, instead) => {
  const code = `LRU_CACHE_OPTION_${opt}`
  if (shouldWarn(code)) {
    warn(code, `${opt} option`, `options.${instead}`, LRUCache)
  }
}
const deprecatedMethod = (method, instead) => {
  const code = `LRU_CACHE_METHOD_${method}`
  if (shouldWarn(code)) {
    const { prototype } = LRUCache
    const { get } = Object.getOwnPropertyDescriptor(prototype, method)
    warn(code, `${method} method`, `cache.${instead}()`, get)
  }
}
const deprecatedProperty = (field, instead) => {
  const code = `LRU_CACHE_PROPERTY_${field}`
  if (shouldWarn(code)) {
    const { prototype } = LRUCache
    const { get } = Object.getOwnPropertyDescriptor(prototype, field)
    warn(code, `${field} property`, `cache.${instead}`, get)
  }
}

const emitWarning = (...a) => {
  typeof process === 'object' &&
  process &&
  typeof process.emitWarning === 'function'
    ? process.emitWarning(...a)
    : console.error(...a)
}

const shouldWarn = code => !warned.has(code)

const warn = (code, what, instead, fn) => {
  warned.add(code)
  const msg = `The ${what} is deprecated. Please use ${instead} instead.`
  emitWarning(msg, 'DeprecationWarning', code, fn)
}

const isPosInt = n => n && n === Math.floor(n) && n > 0 && isFinite(n)

/* istanbul ignore next - This is a little bit ridiculous, tbh.
 * The maximum array length is 2^32-1 or thereabouts on most JS impls.
 * And well before that point, you're caching the entire world, I mean,
 * that's ~32GB of just integers for the next/prev links, plus whatever
 * else to hold that many keys and values.  Just filling the memory with
 * zeroes at init time is brutal when you get that big.
 * But why not be complete?
 * Maybe in the future, these limits will have expanded. */
const getUintArray = max =>
  !isPosInt(max)
    ? null
    : max <= Math.pow(2, 8)
    ? Uint8Array
    : max <= Math.pow(2, 16)
    ? Uint16Array
    : max <= Math.pow(2, 32)
    ? Uint32Array
    : max <= Number.MAX_SAFE_INTEGER
    ? ZeroArray
    : null

class ZeroArray extends Array {
  constructor(size) {
    super(size)
    this.fill(0)
  }
}

class Stack {
  constructor(max) {
    if (max === 0) {
      return []
    }
    const UintArray = getUintArray(max)
    this.heap = new UintArray(max)
    this.length = 0
  }
  push(n) {
    this.heap[this.length++] = n
  }
  pop() {
    return this.heap[--this.length]
  }
}

class LRUCache {
  constructor(options = {}) {
    const {
      max = 0,
      ttl,
      ttlResolution = 1,
      ttlAutopurge,
      updateAgeOnGet,
      updateAgeOnHas,
      allowStale,
      dispose,
      disposeAfter,
      noDisposeOnSet,
      noUpdateTTL,
      maxSize = 0,
      maxEntrySize = 0,
      sizeCalculation,
      fetchMethod,
      fetchContext,
      noDeleteOnFetchRejection,
      noDeleteOnStaleGet,
      allowStaleOnFetchRejection,
      allowStaleOnFetchAbort,
      ignoreFetchAbort,
    } = options

    // deprecated options, don't trigger a warning for getting them if
    // the thing being passed in is another LRUCache we're copying.
    const { length, maxAge, stale } =
      options instanceof LRUCache ? {} : options

    if (max !== 0 && !isPosInt(max)) {
      throw new TypeError('max option must be a nonnegative integer')
    }

    const UintArray = max ? getUintArray(max) : Array
    if (!UintArray) {
      throw new Error('invalid max value: ' + max)
    }

    this.max = max
    this.maxSize = maxSize
    this.maxEntrySize = maxEntrySize || this.maxSize
    this.sizeCalculation = sizeCalculation || length
    if (this.sizeCalculation) {
      if (!this.maxSize && !this.maxEntrySize) {
        throw new TypeError(
          'cannot set sizeCalculation without setting maxSize or maxEntrySize'
        )
      }
      if (typeof this.sizeCalculation !== 'function') {
        throw new TypeError('sizeCalculation set to non-function')
      }
    }

    this.fetchMethod = fetchMethod || null
    if (this.fetchMethod && typeof this.fetchMethod !== 'function') {
      throw new TypeError(
        'fetchMethod must be a function if specified'
      )
    }

    this.fetchContext = fetchContext
    if (!this.fetchMethod && fetchContext !== undefined) {
      throw new TypeError(
        'cannot set fetchContext without fetchMethod'
      )
    }

    this.keyMap = new Map()
    this.keyList = new Array(max).fill(null)
    this.valList = new Array(max).fill(null)
    this.next = new UintArray(max)
    this.prev = new UintArray(max)
    this.head = 0
    this.tail = 0
    this.free = new Stack(max)
    this.initialFill = 1
    this.size = 0

    if (typeof dispose === 'function') {
      this.dispose = dispose
    }
    if (typeof disposeAfter === 'function') {
      this.disposeAfter = disposeAfter
      this.disposed = []
    } else {
      this.disposeAfter = null
      this.disposed = null
    }
    this.noDisposeOnSet = !!noDisposeOnSet
    this.noUpdateTTL = !!noUpdateTTL
    this.noDeleteOnFetchRejection = !!noDeleteOnFetchRejection
    this.allowStaleOnFetchRejection = !!allowStaleOnFetchRejection
    this.allowStaleOnFetchAbort = !!allowStaleOnFetchAbort
    this.ignoreFetchAbort = !!ignoreFetchAbort

    // NB: maxEntrySize is set to maxSize if it's set
    if (this.maxEntrySize !== 0) {
      if (this.maxSize !== 0) {
        if (!isPosInt(this.maxSize)) {
          throw new TypeError(
            'maxSize must be a positive integer if specified'
          )
        }
      }
      if (!isPosInt(this.maxEntrySize)) {
        throw new TypeError(
          'maxEntrySize must be a positive integer if specified'
        )
      }
      this.initializeSizeTracking()
    }

    this.allowStale = !!allowStale || !!stale
    this.noDeleteOnStaleGet = !!noDeleteOnStaleGet
    this.updateAgeOnGet = !!updateAgeOnGet
    this.updateAgeOnHas = !!updateAgeOnHas
    this.ttlResolution =
      isPosInt(ttlResolution) || ttlResolution === 0
        ? ttlResolution
        : 1
    this.ttlAutopurge = !!ttlAutopurge
    this.ttl = ttl || maxAge || 0
    if (this.ttl) {
      if (!isPosInt(this.ttl)) {
        throw new TypeError(
          'ttl must be a positive integer if specified'
        )
      }
      this.initializeTTLTracking()
    }

    // do not allow completely unbounded caches
    if (this.max === 0 && this.ttl === 0 && this.maxSize === 0) {
      throw new TypeError(
        'At least one of max, maxSize, or ttl is required'
      )
    }
    if (!this.ttlAutopurge && !this.max && !this.maxSize) {
      const code = 'LRU_CACHE_UNBOUNDED'
      if (shouldWarn(code)) {
        warned.add(code)
        const msg =
          'TTL caching without ttlAutopurge, max, or maxSize can ' +
          'result in unbounded memory consumption.'
        emitWarning(msg, 'UnboundedCacheWarning', code, LRUCache)
      }
    }

    if (stale) {
      deprecatedOption('stale', 'allowStale')
    }
    if (maxAge) {
      deprecatedOption('maxAge', 'ttl')
    }
    if (length) {
      deprecatedOption('length', 'sizeCalculation')
    }
  }

  getRemainingTTL(key) {
    return this.has(key, { updateAgeOnHas: false }) ? Infinity : 0
  }

  initializeTTLTracking() {
    this.ttls = new ZeroArray(this.max)
    this.starts = new ZeroArray(this.max)

    this.setItemTTL = (index, ttl, start = perf.now()) => {
      this.starts[index] = ttl !== 0 ? start : 0
      this.ttls[index] = ttl
      if (ttl !== 0 && this.ttlAutopurge) {
        const t = setTimeout(() => {
          if (this.isStale(index)) {
            this.delete(this.keyList[index])
          }
        }, ttl + 1)
        /* istanbul ignore else - unref() not supported on all platforms */
        if (t.unref) {
          t.unref()
        }
      }
    }

    this.updateItemAge = index => {
      this.starts[index] = this.ttls[index] !== 0 ? perf.now() : 0
    }

    this.statusTTL = (status, index) => {
      if (status) {
        status.ttl = this.ttls[index]
        status.start = this.starts[index]
        status.now = cachedNow || getNow()
        status.remainingTTL = status.now + status.ttl - status.start
      }
    }

    // debounce calls to perf.now() to 1s so we're not hitting
    // that costly call repeatedly.
    let cachedNow = 0
    const getNow = () => {
      const n = perf.now()
      if (this.ttlResolution > 0) {
        cachedNow = n
        const t = setTimeout(
          () => (cachedNow = 0),
          this.ttlResolution
        )
        /* istanbul ignore else - not available on all platforms */
        if (t.unref) {
          t.unref()
        }
      }
      return n
    }

    this.getRemainingTTL = key => {
      const index = this.keyMap.get(key)
      if (index === undefined) {
        return 0
      }
      return this.ttls[index] === 0 || this.starts[index] === 0
        ? Infinity
        : this.starts[index] +
            this.ttls[index] -
            (cachedNow || getNow())
    }

    this.isStale = index => {
      return (
        this.ttls[index] !== 0 &&
        this.starts[index] !== 0 &&
        (cachedNow || getNow()) - this.starts[index] >
          this.ttls[index]
      )
    }
  }
  updateItemAge(_index) {}
  statusTTL(_status, _index) {}
  setItemTTL(_index, _ttl, _start) {}
  isStale(_index) {
    return false
  }

  initializeSizeTracking() {
    this.calculatedSize = 0
    this.sizes = new ZeroArray(this.max)
    this.removeItemSize = index => {
      this.calculatedSize -= this.sizes[index]
      this.sizes[index] = 0
    }
    this.requireSize = (k, v, size, sizeCalculation) => {
      // provisionally accept background fetches.
      // actual value size will be checked when they return.
      if (this.isBackgroundFetch(v)) {
        return 0
      }
      if (!isPosInt(size)) {
        if (sizeCalculation) {
          if (typeof sizeCalculation !== 'function') {
            throw new TypeError('sizeCalculation must be a function')
          }
          size = sizeCalculation(v, k)
          if (!isPosInt(size)) {
            throw new TypeError(
              'sizeCalculation return invalid (expect positive integer)'
            )
          }
        } else {
          throw new TypeError(
            'invalid size value (must be positive integer). ' +
              'When maxSize or maxEntrySize is used, sizeCalculation or size ' +
              'must be set.'
          )
        }
      }
      return size
    }
    this.addItemSize = (index, size, status) => {
      this.sizes[index] = size
      if (this.maxSize) {
        const maxSize = this.maxSize - this.sizes[index]
        while (this.calculatedSize > maxSize) {
          this.evict(true)
        }
      }
      this.calculatedSize += this.sizes[index]
      if (status) {
        status.entrySize = size
        status.totalCalculatedSize = this.calculatedSize
      }
    }
  }
  removeItemSize(_index) {}
  addItemSize(_index, _size) {}
  requireSize(_k, _v, size, sizeCalculation) {
    if (size || sizeCalculation) {
      throw new TypeError(
        'cannot set size without setting maxSize or maxEntrySize on cache'
      )
    }
  }

  *indexes({ allowStale = this.allowStale } = {}) {
    if (this.size) {
      for (let i = this.tail; true; ) {
        if (!this.isValidIndex(i)) {
          break
        }
        if (allowStale || !this.isStale(i)) {
          yield i
        }
        if (i === this.head) {
          break
        } else {
          i = this.prev[i]
        }
      }
    }
  }

  *rindexes({ allowStale = this.allowStale } = {}) {
    if (this.size) {
      for (let i = this.head; true; ) {
        if (!this.isValidIndex(i)) {
          break
        }
        if (allowStale || !this.isStale(i)) {
          yield i
        }
        if (i === this.tail) {
          break
        } else {
          i = this.next[i]
        }
      }
    }
  }

  isValidIndex(index) {
    return (
      index !== undefined &&
      this.keyMap.get(this.keyList[index]) === index
    )
  }

  *entries() {
    for (const i of this.indexes()) {
      if (
        this.valList[i] !== undefined &&
        this.keyList[i] !== undefined &&
        !this.isBackgroundFetch(this.valList[i])
      ) {
        yield [this.keyList[i], this.valList[i]]
      }
    }
  }
  *rentries() {
    for (const i of this.rindexes()) {
      if (
        this.valList[i] !== undefined &&
        this.keyList[i] !== undefined &&
        !this.isBackgroundFetch(this.valList[i])
      ) {
        yield [this.keyList[i], this.valList[i]]
      }
    }
  }

  *keys() {
    for (const i of this.indexes()) {
      if (
        this.keyList[i] !== undefined &&
        !this.isBackgroundFetch(this.valList[i])
      ) {
        yield this.keyList[i]
      }
    }
  }
  *rkeys() {
    for (const i of this.rindexes()) {
      if (
        this.keyList[i] !== undefined &&
        !this.isBackgroundFetch(this.valList[i])
      ) {
        yield this.keyList[i]
      }
    }
  }

  *values() {
    for (const i of this.indexes()) {
      if (
        this.valList[i] !== undefined &&
        !this.isBackgroundFetch(this.valList[i])
      ) {
        yield this.valList[i]
      }
    }
  }
  *rvalues() {
    for (const i of this.rindexes()) {
      if (
        this.valList[i] !== undefined &&
        !this.isBackgroundFetch(this.valList[i])
      ) {
        yield this.valList[i]
      }
    }
  }

  [Symbol.iterator]() {
    return this.entries()
  }

  find(fn, getOptions) {
    for (const i of this.indexes()) {
      const v = this.valList[i]
      const value = this.isBackgroundFetch(v)
        ? v.__staleWhileFetching
        : v
      if (value === undefined) continue
      if (fn(value, this.keyList[i], this)) {
        return this.get(this.keyList[i], getOptions)
      }
    }
  }

  forEach(fn, thisp = this) {
    for (const i of this.indexes()) {
      const v = this.valList[i]
      const value = this.isBackgroundFetch(v)
        ? v.__staleWhileFetching
        : v
      if (value === undefined) continue
      fn.call(thisp, value, this.keyList[i], this)
    }
  }

  rforEach(fn, thisp = this) {
    for (const i of this.rindexes()) {
      const v = this.valList[i]
      const value = this.isBackgroundFetch(v)
        ? v.__staleWhileFetching
        : v
      if (value === undefined) continue
      fn.call(thisp, value, this.keyList[i], this)
    }
  }

  get prune() {
    deprecatedMethod('prune', 'purgeStale')
    return this.purgeStale
  }

  purgeStale() {
    let deleted = false
    for (const i of this.rindexes({ allowStale: true })) {
      if (this.isStale(i)) {
        this.delete(this.keyList[i])
        deleted = true
      }
    }
    return deleted
  }

  dump() {
    const arr = []
    for (const i of this.indexes({ allowStale: true })) {
      const key = this.keyList[i]
      const v = this.valList[i]
      const value = this.isBackgroundFetch(v)
        ? v.__staleWhileFetching
        : v
      if (value === undefined) continue
      const entry = { value }
      if (this.ttls) {
        entry.ttl = this.ttls[i]
        // always dump the start relative to a portable timestamp
        // it's ok for this to be a bit slow, it's a rare operation.
        const age = perf.now() - this.starts[i]
        entry.start = Math.floor(Date.now() - age)
      }
      if (this.sizes) {
        entry.size = this.sizes[i]
      }
      arr.unshift([key, entry])
    }
    return arr
  }

  load(arr) {
    this.clear()
    for (const [key, entry] of arr) {
      if (entry.start) {
        // entry.start is a portable timestamp, but we may be using
        // node's performance.now(), so calculate the offset.
        // it's ok for this to be a bit slow, it's a rare operation.
        const age = Date.now() - entry.start
        entry.start = perf.now() - age
      }
      this.set(key, entry.value, entry)
    }
  }

  dispose(_v, _k, _reason) {}

  set(
    k,
    v,
    {
      ttl = this.ttl,
      start,
      noDisposeOnSet = this.noDisposeOnSet,
      size = 0,
      sizeCalculation = this.sizeCalculation,
      noUpdateTTL = this.noUpdateTTL,
      status,
    } = {}
  ) {
    size = this.requireSize(k, v, size, sizeCalculation)
    // if the item doesn't fit, don't do anything
    // NB: maxEntrySize set to maxSize by default
    if (this.maxEntrySize && size > this.maxEntrySize) {
      if (status) {
        status.set = 'miss'
        status.maxEntrySizeExceeded = true
      }
      // have to delete, in case a background fetch is there already.
      // in non-async cases, this is a no-op
      this.delete(k)
      return this
    }
    let index = this.size === 0 ? undefined : this.keyMap.get(k)
    if (index === undefined) {
      // addition
      index = this.newIndex()
      this.keyList[index] = k
      this.valList[index] = v
      this.keyMap.set(k, index)
      this.next[this.tail] = index
      this.prev[index] = this.tail
      this.tail = index
      this.size++
      this.addItemSize(index, size, status)
      if (status) {
        status.set = 'add'
      }
      noUpdateTTL = false
    } else {
      // update
      this.moveToTail(index)
      const oldVal = this.valList[index]
      if (v !== oldVal) {
        if (this.isBackgroundFetch(oldVal)) {
          oldVal.__abortController.abort(new Error('replaced'))
        } else {
          if (!noDisposeOnSet) {
            this.dispose(oldVal, k, 'set')
            if (this.disposeAfter) {
              this.disposed.push([oldVal, k, 'set'])
            }
          }
        }
        this.removeItemSize(index)
        this.valList[index] = v
        this.addItemSize(index, size, status)
        if (status) {
          status.set = 'replace'
          const oldValue =
            oldVal && this.isBackgroundFetch(oldVal)
              ? oldVal.__staleWhileFetching
              : oldVal
          if (oldValue !== undefined) status.oldValue = oldValue
        }
      } else if (status) {
        status.set = 'update'
      }
    }
    if (ttl !== 0 && this.ttl === 0 && !this.ttls) {
      this.initializeTTLTracking()
    }
    if (!noUpdateTTL) {
      this.setItemTTL(index, ttl, start)
    }
    this.statusTTL(status, index)
    if (this.disposeAfter) {
      while (this.disposed.length) {
        this.disposeAfter(...this.disposed.shift())
      }
    }
    return this
  }

  newIndex() {
    if (this.size === 0) {
      return this.tail
    }
    if (this.size === this.max && this.max !== 0) {
      return this.evict(false)
    }
    if (this.free.length !== 0) {
      return this.free.pop()
    }
    // initial fill, just keep writing down the list
    return this.initialFill++
  }

  pop() {
    if (this.size) {
      const val = this.valList[this.head]
      this.evict(true)
      return val
    }
  }

  evict(free) {
    const head = this.head
    const k = this.keyList[head]
    const v = this.valList[head]
    if (this.isBackgroundFetch(v)) {
      v.__abortController.abort(new Error('evicted'))
    } else {
      this.dispose(v, k, 'evict')
      if (this.disposeAfter) {
        this.disposed.push([v, k, 'evict'])
      }
    }
    this.removeItemSize(head)
    // if we aren't about to use the index, then null these out
    if (free) {
      this.keyList[head] = null
      this.valList[head] = null
      this.free.push(head)
    }
    this.head = this.next[head]
    this.keyMap.delete(k)
    this.size--
    return head
  }

  has(k, { updateAgeOnHas = this.updateAgeOnHas, status } = {}) {
    const index = this.keyMap.get(k)
    if (index !== undefined) {
      if (!this.isStale(index)) {
        if (updateAgeOnHas) {
          this.updateItemAge(index)
        }
        if (status) status.has = 'hit'
        this.statusTTL(status, index)
        return true
      } else if (status) {
        status.has = 'stale'
        this.statusTTL(status, index)
      }
    } else if (status) {
      status.has = 'miss'
    }
    return false
  }

  // like get(), but without any LRU updating or TTL expiration
  peek(k, { allowStale = this.allowStale } = {}) {
    const index = this.keyMap.get(k)
    if (index !== undefined && (allowStale || !this.isStale(index))) {
      const v = this.valList[index]
      // either stale and allowed, or forcing a refresh of non-stale value
      return this.isBackgroundFetch(v) ? v.__staleWhileFetching : v
    }
  }

  backgroundFetch(k, index, options, context) {
    const v = index === undefined ? undefined : this.valList[index]
    if (this.isBackgroundFetch(v)) {
      return v
    }
    const ac = new AC()
    if (options.signal) {
      options.signal.addEventListener('abort', () =>
        ac.abort(options.signal.reason)
      )
    }
    const fetchOpts = {
      signal: ac.signal,
      options,
      context,
    }
    const cb = (v, updateCache = false) => {
      const { aborted } = ac.signal
      const ignoreAbort = options.ignoreFetchAbort && v !== undefined
      if (options.status) {
        if (aborted && !updateCache) {
          options.status.fetchAborted = true
          options.status.fetchError = ac.signal.reason
          if (ignoreAbort) options.status.fetchAbortIgnored = true
        } else {
          options.status.fetchResolved = true
        }
      }
      if (aborted && !ignoreAbort && !updateCache) {
        return fetchFail(ac.signal.reason)
      }
      // either we didn't abort, and are still here, or we did, and ignored
      if (this.valList[index] === p) {
        if (v === undefined) {
          if (p.__staleWhileFetching) {
            this.valList[index] = p.__staleWhileFetching
          } else {
            this.delete(k)
          }
        } else {
          if (options.status) options.status.fetchUpdated = true
          this.set(k, v, fetchOpts.options)
        }
      }
      return v
    }
    const eb = er => {
      if (options.status) {
        options.status.fetchRejected = true
        options.status.fetchError = er
      }
      return fetchFail(er)
    }
    const fetchFail = er => {
      const { aborted } = ac.signal
      const allowStaleAborted =
        aborted && options.allowStaleOnFetchAbort
      const allowStale =
        allowStaleAborted || options.allowStaleOnFetchRejection
      const noDelete = allowStale || options.noDeleteOnFetchRejection
      if (this.valList[index] === p) {
        // if we allow stale on fetch rejections, then we need to ensure that
        // the stale value is not removed from the cache when the fetch fails.
        const del = !noDelete || p.__staleWhileFetching === undefined
        if (del) {
          this.delete(k)
        } else if (!allowStaleAborted) {
          // still replace the *promise* with the stale value,
          // since we are done with the promise at this point.
          // leave it untouched if we're still waiting for an
          // aborted background fetch that hasn't yet returned.
          this.valList[index] = p.__staleWhileFetching
        }
      }
      if (allowStale) {
        if (options.status && p.__staleWhileFetching !== undefined) {
          options.status.returnedStale = true
        }
        return p.__staleWhileFetching
      } else if (p.__returned === p) {
        throw er
      }
    }
    const pcall = (res, rej) => {
      this.fetchMethod(k, v, fetchOpts).then(v => res(v), rej)
      // ignored, we go until we finish, regardless.
      // defer check until we are actually aborting,
      // so fetchMethod can override.
      ac.signal.addEventListener('abort', () => {
        if (
          !options.ignoreFetchAbort ||
          options.allowStaleOnFetchAbort
        ) {
          res()
          // when it eventually resolves, update the cache.
          if (options.allowStaleOnFetchAbort) {
            res = v => cb(v, true)
          }
        }
      })
    }
    if (options.status) options.status.fetchDispatched = true
    const p = new Promise(pcall).then(cb, eb)
    p.__abortController = ac
    p.__staleWhileFetching = v
    p.__returned = null
    if (index === undefined) {
      // internal, don't expose status.
      this.set(k, p, { ...fetchOpts.options, status: undefined })
      index = this.keyMap.get(k)
    } else {
      this.valList[index] = p
    }
    return p
  }

  isBackgroundFetch(p) {
    return (
      p &&
      typeof p === 'object' &&
      typeof p.then === 'function' &&
      Object.prototype.hasOwnProperty.call(
        p,
        '__staleWhileFetching'
      ) &&
      Object.prototype.hasOwnProperty.call(p, '__returned') &&
      (p.__returned === p || p.__returned === null)
    )
  }

  // this takes the union of get() and set() opts, because it does both
  async fetch(
    k,
    {
      // get options
      allowStale = this.allowStale,
      updateAgeOnGet = this.updateAgeOnGet,
      noDeleteOnStaleGet = this.noDeleteOnStaleGet,
      // set options
      ttl = this.ttl,
      noDisposeOnSet = this.noDisposeOnSet,
      size = 0,
      sizeCalculation = this.sizeCalculation,
      noUpdateTTL = this.noUpdateTTL,
      // fetch exclusive options
      noDeleteOnFetchRejection = this.noDeleteOnFetchRejection,
      allowStaleOnFetchRejection = this.allowStaleOnFetchRejection,
      ignoreFetchAbort = this.ignoreFetchAbort,
      allowStaleOnFetchAbort = this.allowStaleOnFetchAbort,
      fetchContext = this.fetchContext,
      forceRefresh = false,
      status,
      signal,
    } = {}
  ) {
    if (!this.fetchMethod) {
      if (status) status.fetch = 'get'
      return this.get(k, {
        allowStale,
        updateAgeOnGet,
        noDeleteOnStaleGet,
        status,
      })
    }

    const options = {
      allowStale,
      updateAgeOnGet,
      noDeleteOnStaleGet,
      ttl,
      noDisposeOnSet,
      size,
      sizeCalculation,
      noUpdateTTL,
      noDeleteOnFetchRejection,
      allowStaleOnFetchRejection,
      allowStaleOnFetchAbort,
      ignoreFetchAbort,
      status,
      signal,
    }

    let index = this.keyMap.get(k)
    if (index === undefined) {
      if (status) status.fetch = 'miss'
      const p = this.backgroundFetch(k, index, options, fetchContext)
      return (p.__returned = p)
    } else {
      // in cache, maybe already fetching
      const v = this.valList[index]
      if (this.isBackgroundFetch(v)) {
        const stale =
          allowStale && v.__staleWhileFetching !== undefined
        if (status) {
          status.fetch = 'inflight'
          if (stale) status.returnedStale = true
        }
        return stale ? v.__staleWhileFetching : (v.__returned = v)
      }

      // if we force a refresh, that means do NOT serve the cached value,
      // unless we are already in the process of refreshing the cache.
      const isStale = this.isStale(index)
      if (!forceRefresh && !isStale) {
        if (status) status.fetch = 'hit'
        this.moveToTail(index)
        if (updateAgeOnGet) {
          this.updateItemAge(index)
        }
        this.statusTTL(status, index)
        return v
      }

      // ok, it is stale or a forced refresh, and not already fetching.
      // refresh the cache.
      const p = this.backgroundFetch(k, index, options, fetchContext)
      const hasStale = p.__staleWhileFetching !== undefined
      const staleVal = hasStale && allowStale
      if (status) {
        status.fetch = hasStale && isStale ? 'stale' : 'refresh'
        if (staleVal && isStale) status.returnedStale = true
      }
      return staleVal ? p.__staleWhileFetching : (p.__returned = p)
    }
  }

  get(
    k,
    {
      allowStale = this.allowStale,
      updateAgeOnGet = this.updateAgeOnGet,
      noDeleteOnStaleGet = this.noDeleteOnStaleGet,
      status,
    } = {}
  ) {
    const index = this.keyMap.get(k)
    if (index !== undefined) {
      const value = this.valList[index]
      const fetching = this.isBackgroundFetch(value)
      this.statusTTL(status, index)
      if (this.isStale(index)) {
        if (status) status.get = 'stale'
        // delete only if not an in-flight background fetch
        if (!fetching) {
          if (!noDeleteOnStaleGet) {
            this.delete(k)
          }
          if (status) status.returnedStale = allowStale
          return allowStale ? value : undefined
        } else {
          if (status) {
            status.returnedStale =
              allowStale && value.__staleWhileFetching !== undefined
          }
          return allowStale ? value.__staleWhileFetching : undefined
        }
      } else {
        if (status) status.get = 'hit'
        // if we're currently fetching it, we don't actually have it yet
        // it's not stale, which means this isn't a staleWhileRefetching.
        // If it's not stale, and fetching, AND has a __staleWhileFetching
        // value, then that means the user fetched with {forceRefresh:true},
        // so it's safe to return that value.
        if (fetching) {
          return value.__staleWhileFetching
        }
        this.moveToTail(index)
        if (updateAgeOnGet) {
          this.updateItemAge(index)
        }
        return value
      }
    } else if (status) {
      status.get = 'miss'
    }
  }

  connect(p, n) {
    this.prev[n] = p
    this.next[p] = n
  }

  moveToTail(index) {
    // if tail already, nothing to do
    // if head, move head to next[index]
    // else
    //   move next[prev[index]] to next[index] (head has no prev)
    //   move prev[next[index]] to prev[index]
    // prev[index] = tail
    // next[tail] = index
    // tail = index
    if (index !== this.tail) {
      if (index === this.head) {
        this.head = this.next[index]
      } else {
        this.connect(this.prev[index], this.next[index])
      }
      this.connect(this.tail, index)
      this.tail = index
    }
  }

  get del() {
    deprecatedMethod('del', 'delete')
    return this.delete
  }

  delete(k) {
    let deleted = false
    if (this.size !== 0) {
      const index = this.keyMap.get(k)
      if (index !== undefined) {
        deleted = true
        if (this.size === 1) {
          this.clear()
        } else {
          this.removeItemSize(index)
          const v = this.valList[index]
          if (this.isBackgroundFetch(v)) {
            v.__abortController.abort(new Error('deleted'))
          } else {
            this.dispose(v, k, 'delete')
            if (this.disposeAfter) {
              this.disposed.push([v, k, 'delete'])
            }
          }
          this.keyMap.delete(k)
          this.keyList[index] = null
          this.valList[index] = null
          if (index === this.tail) {
            this.tail = this.prev[index]
          } else if (index === this.head) {
            this.head = this.next[index]
          } else {
            this.next[this.prev[index]] = this.next[index]
            this.prev[this.next[index]] = this.prev[index]
          }
          this.size--
          this.free.push(index)
        }
      }
    }
    if (this.disposed) {
      while (this.disposed.length) {
        this.disposeAfter(...this.disposed.shift())
      }
    }
    return deleted
  }

  clear() {
    for (const index of this.rindexes({ allowStale: true })) {
      const v = this.valList[index]
      if (this.isBackgroundFetch(v)) {
        v.__abortController.abort(new Error('deleted'))
      } else {
        const k = this.keyList[index]
        this.dispose(v, k, 'delete')
        if (this.disposeAfter) {
          this.disposed.push([v, k, 'delete'])
        }
      }
    }

    this.keyMap.clear()
    this.valList.fill(null)
    this.keyList.fill(null)
    if (this.ttls) {
      this.ttls.fill(0)
      this.starts.fill(0)
    }
    if (this.sizes) {
      this.sizes.fill(0)
    }
    this.head = 0
    this.tail = 0
    this.initialFill = 1
    this.free.length = 0
    this.calculatedSize = 0
    this.size = 0
    if (this.disposed) {
      while (this.disposed.length) {
        this.disposeAfter(...this.disposed.shift())
      }
    }
  }

  get reset() {
    deprecatedMethod('reset', 'clear')
    return this.clear
  }

  get length() {
    deprecatedProperty('length', 'size')
    return this.size
  }

  static get AbortController() {
    return AC
  }
  static get AbortSignal() {
    return AS
  }
}

module.exports = LRUCache